The present inventors already have a granted patent of a modified metal oxide sol having a sulfonic acid group (Patent document 1). Although the substrate (glass, plastic, metal, etc.) applied by a hydrophilic coating solution comprising the traditional modified metal oxide sol shows hydrophilic property, it does not show antifog effect. And when the lens treated by the coating solution is exposed to steam, water drops generate and sight becomes poor.
As the patent of the antifog additive, an antifog additive using phosphoric ester-type emulsifier (Patent document 2) and an antifog additive using polyacrylic acids (Patent document 3) have been filed.